Why is smoking forbidden in Ramadan?

From our friends at Islam Q&A:

Smoking is haraam and there is no doubt that it is forbidden...

With regard to the reason why it breaks the fast, that is because the smoke contains particles that reach the stomach.

Shaykh Ibn ‘Uthaymeen was asked about the fasting person smelling perfume and he said:

It is permissible to use it during the day in Ramadaan and to smell it, except for bukhoor (incense), because incense contains particles that can reach the stomach, and that is smoke.

Fataawa Islamiyyah, 2/128.

Cigarette smoke is like incense in that they both contain particles, but they differ with regard to the basic ruling: incense is halaal and good, and smoking is haraam and bad.

And Allaah knows best.